When drilling an oil well or a well for another effluent (in particular gas or water), it is required to accurately monitor the flow of displaced drilling fluids or muds.
The muds circulating in a wellbore during drilling are generally injected from, and returned to, several pits hydraulically connected to each other. The hydraulic connections may be reversible, by operation of valves, in order to control the actual involvement of each pit in the mud supply/recovery in and out of the wellbore.
Fluid gain or loss during drilling may lead to important problems for the further exploitation of the well. However, due to the important volumes of the mud pits, it is difficult to control the volume variations for detecting the gains or losses.
It is known to provide surface sensors to measure the evolution of the mud volume of each of the pits. Due to the fluid transfer between the different pits, it is however not straightforward to derive information about gains and losses from the measurements of the surface sensors.